The synchronous optical network (SONET) standards for optical transmission include a number of mechanisms and recommendations for protecting transmitted traffic. These include making use of “rings” that contain all of the transmission termination points. Such rings provide two paths between each transmission point on the ring, thus providing a level of redundancy. There are two basic ways to exploit SONET rings to protect traffic: a uni-directional path switched ring (UPSR) and a bi-directional line switched ring (BLSR).
A UPSR operates by having two copies of the same traffic go in opposite directions from one side of a SONET ring to the other. At the exit point from the ring, the traffic from the two directions is compared, and the better version is selected for output. This approach can be utilized for time division multiplexed (TDM) traffic using virtual tributary (VT) comparison. Specifically, the overhead associated with each VT that provides performance indications is inspected to determine which path exhibits better performance by whatever performance criteria is chosen.
Traditional approaches to UPSR for TDM involve switch reconfiguration. Specifically, a TDM switch is provided at the ring node where the VT is being extracted from the ring. The TDM switch is configured to switch the TDM traffic from one of the two VT copies through to a destination. When the VT performance measures indicate that the VT copy being discarded has become the superior or preferred version of the two VT copies, the TDM switch is reconfigured to switch through this VT copy and to ignore the original VT copy. Such reconfigurations can be carried out indefinitely.
This type of TDM switch reconfiguration approach severely taxes the processing resources, such as a microprocessor (or other computing entity) that is controlling the switch. Reconfiguration is often a computationally involved task. The SONET standards require that switch reconfiguration occur within 50 ms of the failure of a transmission link that is carrying the active VT copy. This standard may not always be met under certain conditions.